This invention relates generally to memory elements and more particularly to random-access memory elements.
As is known in the art, in a random-access memory information can be read from any random address in the memory or written into any random address in the memory with similar read cycle and write cycle times. There are two general types of random-access memory. The first type of random-access memory is so-called semiconductor random-access memory in which active devices such as transistors are arranged to provide memory storage elements. Semiconductor memories are generally volatile, that is information is lost if power is interrupted, and radiation-soft, that is information is lost if the memory is subjected to ionizing radiation or an electromagnetic pulse beyond certain threshold tolerances. Threshold tolerances depend upon the type of memory elements and the material from which the memory elements are formed. For many applications, semiconductor memories are nevertheless very important. Semiconductor memories are very fast, typically having access times of several nanoseconds and faster, and are relatively inexpensive because the independent memory cells may be packed very close together and, therefore, the information density or the number of bits per unit area is very high. Also, semiconductor memories are fabricated from relatively inexpensive materials using integrated circuit fabrication techniques which further reduces the cost of the memory.
The second type of memory is generally referred to as magnetic memories. With magnetic type memories, the independent storage element or memory cell is a magnetic material or device. Two types of known magnetic memories are ferrite core memories and plated wire memories. These memories are generally non-volatile and radiation-hard. Accordingly, these devices are used in applications where non-volatility is required, as well as, in high radiation environments. However, these memories are also extremely slow and expensive. They are expensive because they cannot be fabricated by inexpensive techniques such as integrated circuit fabrication techniques and their information density is very low particularly when compared to that of semiconductor memories.
Nevertheless, there are many applications which could use a random-access, high density, fast memory, that is radiation-hard and non-volatile. Several approaches are used to compensate for the lack of non-volatile semiconductor RAM. For example, in commercial computer applications, memory elements called "electrical alterable read-only memories" (EAROM) and others known as erasable programmable read only memories EPROM are widely used, often in applications more suitable for non-volatile random-access memory. Neither of these memory types are random access because they may require an erase cycle prior to altering data in the memory elements, and generally the write cycle time is much slower by an order of magnitude or more, than the read cycle time. Further, these memory types are nevertheless "radiation soft" , susceptible to upset due to ionizing radiation. Accordingly, these memory types are unacceptable substitutes for magnetic core and plated wire memories in system which may be exposed to high radiation or electromagnetic pulse environments. Therefore, the lack of non-volatility and in particular, radiation-hardness are important considerations which mitigate against the use of a semiconductor memory in certain applications to replace the known types of magnetic memories in high radiation exposure environments.
One memory system described in U.S. Pat. No. 3,573,485, issued to Ballard involves a passive element device such as a ferromagnetic core which is disposed in the path of a semiconductor flip-flop. In previous magnetic core memories, one of the problems associated therewith was that readout from the core was generally destructive, that is when information was read from a particular magnetic core element, the information stored was erased and, therefore, circuits had to be provided which would restore the information to the memory element. U.S. Pat. No. '485 described a technique in which the magnetic storage device such as the core was disposed in the cross-coupled paths of a semiconductor flip-flop which prevented erasure of the core during a read cycle. While this technique represented an improvement to existing core memory, the technique was not sufficiently adaptable to provide a new type of memory, to replace semiconductor RAM memory in high information density applications. The use of a magnetic core did not solve the problems of the slow speed, or expense, since the core is not amenable to high packing density. Furthermore, the magnetic core described in U.S. Pat. No. '485 uses a magnetic element in which remanent magnetizations used for information storage is in either of two opposite polarities. With this arrangement, the process of magnetization reversal proceeds by a mechanism known as domain-wall motion. Domain wall motion, however, is ineffective at higher frequencies and, accordingly these devices are inherently slow. If the magnetic storage device is used in combination with semiconductor flip-flop circuits, it is important to have the magnetic storage device operate with a relatively high or fast response time, comparable to that of semiconductor circuit, typically in the order of 100's of pico seconds to several nanoseconds. Magnetization reversal by means of domain-wall motion is too slow to be significant at these response times.